A traditional method of brewing soy sauce comprises admixing steamed soybean products with roasted and milled wheat, inoculating the mixture with seed koji mold for soy sauce, and culturing the mixture to obtain soy sauce koji, adding an appropriate amount of a salt water to the soy sauce koji to obtain a soy sauce moromi mash, fermenting and aging for a certain time period the moromi mash to obtain a aged moromi mash, and finally press-filtering the aged moromi mash. However, the fermentation and aging of a soy sauce moromi mash are normally performed in an open system, and almost no microorganism, such as soy sauce yeast or soy sauce lactic acid bacteria, are artificially added. Hence, all of the microorganisms (yeast and lactic acid bacteria) acting in the moromi mash are derived by natural contamination from the natural flora inhabiting the brewing sites, facilities, instruments or the like.
In the present invention, the term “wild yeast” indicates yeast which inhabits sites, facilities, instruments and air and the like for brewing soy sauce, and naturally contaminates and proliferates in a soy sauce moromi mash.
In contrast soy sauce yeast used in the present invention is easily collected (separated) from a soy sauce moromi mash and the like by a known ordinary means, and preferably is highly capable of producing alcohol and being salt-tolerant. The soy sauce yeast is pure cultured artificially and added to a soy sauce moromi mash. The thus pre-cultured soy sauce yeast is clearly distinguished horn wild yeast.
The natural flora is greatly diversified, so that naturally contaminated soy sauce yeast and soy sauce lactic acid bacteria may contain bacteria having properties that are not exactly favorable in terms of soy sauce quality. Further, there are disadvantages due to the differences in the composition of the natural flora depending on area or with time, such that the product quality varies depending on the brewing site, and that soy sauce having uniform quality cannot always be obtained throughout the year.
From these viewpoints, to constantly produce brewed soy sauce having a uniform quality regardless of area or time constraints, a method of brewing soy sauce has been developed. This method of brewing soy sauce comprises mixing soy sauce koji into a salt water to prepare a soy sauce moromi mash; adding pre-cultured soy sauce lactic acid bacteria and soy sauce yeast to the moromi mash; and then performing fermentation and aging of the moromi mash after adding by ordinary techniques.
Specifically, a known method comprises mixing soy sauce koji into a salt water to prepare a soy sauce moromi mash, adding pre-cultured soy sauce lactic acid bacteria to the moromi mash, culturing the bacteria, adding soy sauce yeast (mainly Zygosaccharomyces rouxii, the main soy sauce fermentation yeast) when the cell count of the lactic acid bacteria in the moromi mash exceeds 107 cells/g moromi and the pH of the moromi decreases to 5.1 to 5.2 as lactic acid fermentation proceeds (normally at 20 to 40 days after mixing), performing ethanol fermentation, and thus brewing soy sauce having good flavor (Science and Technique of Soy Sauce, edited and written by Shinrokuro TOCHIKURA, BREWING SOCIETY OF JAPAN, 1988) (food Microbiology Hand Book, edited and written by Hisao YOSHII, Yasuyuki KANEKO and Kazuo YAMAGUCHI, Giho-do, 1995).
However in this method, since non-sterile koji is used and fermentation and aging arc performed in a microbiologically open vessel, lactic acid fermentation and yeast fermentation for a soy sauce moromi mash is easily contaminated with wild microorganisms due to complex troubling factors that cannot be specified. Thus, it is very difficult to maintain a stable and balanced growth of these added microorganisms over a long period while maintaining preferred fermentation.
Moreover under pure culture conditions, there is a disadvantage that although the main soy sauce fermentation yeast strains having high ethanol productivity or being capable of forming good soy sauce aroma are obtained when these strains are positively used in an actual brewing process, results that can be obtained under a pure culture condition cannot be obtained in many cases.
Accordingly, in recent years, efforts have been made to achieve balanced and stable long term growth of the added soy sauce yeast and lactic acid bacteria, so as to maintain preferred yeast fermentation and lactic acid fermentation. Specifically, fermentation and aging of a moromi mash are controlled in the latest, sealed and independent type mixing and fermentation tanks to prevent scattering or contaminating the moromi mash within each mixing and fermentation tank and among adjacent mixing and fermentation tanks, and mixing and fermentation tanks or mixing and fermentation pathways (paths) are cleaned after every use.
However, this method also has a disadvantage because of unstable pH decreases associated with lactic acid fermentation due to, for example, different lots of soy sauce koji or moromi mashes differing from each other, so that there is no other choice but to judge a good timing for addition of yeast depending on the experience and feel of experts.
Further, there is a limit to completely eliminate microorganisms from cleaned mixing and fermentation tanks or mixing and fermentation pathways. Thus, there is a disadvantage that when salt-tolerant wild yeast cannot be eliminated, small numbers of this wild yeast may remain and contaminate a soy sauce moromi mash, and may grow to be dominant before the addition of soy sauce yeast, thus not only is lactic acid fermentation suppressed, but also unfavorable odor is generated by the wild yeast.
Furthermore, yeast to be added should have been previously cultured to a required quantity and stored. There is a disadvantage that when an expected timing for addition of yeast is significantly varied earlier or later the predicted time, shortages of yeast to be added or decreased yeast activity may result, so that yeast fermentation is not performed immediately after addition to a moromi mash.